Commercial analog cellular telephone systems operate in a full-duplex mode, i.e. forward communication from the base station to the subscriber unit on one channel is carried on simultaneously with reverse communication from the subscriber unit to the base station on another channel. Consequently, the subscriber unit must have duplexer circuitry supporting simultaneous transmission and reception.
The commercial cellular telephone network lends itself ideally to the transmission of data from numerous unattended sensors (such as, for example, utility meters) to a central location. The cellular transceivers used by subscribers for this purpose are high-volume items for which cost is a significant factor. Inasmuch as telemetry data is only being communicated in one direction and the volume of data is small, half-duplex operation is sufficient. This in turn allows substantial cost savings for the telemetry units by omitting the duplexer circuitry.
Analog cellular telephony operates in accordance with a standard known as EIA/TIA-553. This standard requires the subscriber unit, while communicating on the reverse channel, to monitor the status of the reverse channel (which is shared by many subscriber units) via the continuously transmitting forward channel. This status is indicated by a busy-idle bit inserted into the pulse train, or bit stream, of the forward channel after every tenth or eleventh bit. When a call from the subscriber unit is initiated, the subscriber unit transmits an identifying signal on the reverse channel and starts a bit count. If the reverse channel goes busy before the 56th bit is transmitted by the subscriber unit, data transmission from the subscriber on the reverse channel is aborted because, presumably, another unit has seized the reverse channel. Conversely, if the reverse channel remains idle after 104 bits have been transmitted by the subscriber unit, subscriber transmission is aborted because the connection has failed.
It is therefore necessary, if half duplex operation is desired, to monitor the forward channel status bits while transmitting on the reverse channel, without using any duplexing circuitry. This has, so far, not been possible in the prior art.